In recent years, faster access networks have been developing rapidly, and FTTH (Fiber To The Home) using optical fibers has been coming into widespread use. Optical fiber installation configurations for achieving FTTH fall into three broad categories: (1) SS (Single Star) type networks in which the station of a common carrier and users are connected by an optical fiber with a one-to-one correspondence; (2) ADS (Active Double Star) type networks in which an active element RT (Remote Terminal) for multiplexing and separating signals and transforming signals electrically and optically is installed between a common carrier and users so that the common carrier and the RT are connected by an optical fiber, and the RT and users are connected by a metal cable with a one-to-one correspondence; and (3) PDS (Passive Double Star) type networks (global standard name PON: Passive Optical Network) in which a light splitter SC (Star Coupler) is installed between a common carrier and users, a light signal from the station is branched in the SC, and optical fibers are installed between the SC and the users.
The PON type networks are lower in fiber installation cost than the SS type networks in which all users and the station are connected with each other by an optical fiber with a one-to-one correspondence. Also, the SC for branching light is a highly reliable passive element and requires no electrical supply, therefore not requiring a power facility and provision for power failure. For these reasons, the PON type networks are very promising technology for achieving FTTH.
Referring to FIG. 1, a description is made of BPON (Broadband optical access systems based on Passive Optical Network), which is PON using ATM (Asynchronous Transfer Mode). The BPON comprises an optical line termination (OLT) 10, optical network units (ONU) 12, and an optical star coupler (SC) 11 (hereinafter, a PON configuration including OLT, SC, and ONU will be referred to as a PON system). The OLT 10, which is primarily installed in a building or the like of a common carrier, makes authorization and bandwidth management for the ONUs 12 in the PON system. The ONUs 12 terminating a user network transform user packets received from users to ATM cells and output as many ATM cells as specified time slots at a timing specified by the OLT 10. The SC 11, which is a passive element constructed of an optical fiber, branches an optical fiber 16 of the OLT side to plural optical fibers 17-1, 17-2, . . . , and 17-n of the ONU side. User data sent from the OLT 10 are sent to all ONUs via the SC 11. The SC 11 multiplexes upstream user packets delivered from the ONUs 12 and outputs the multiplexed packets to the OLT 10. Time slot assignment to the ONUs is made by an OpS (Operation System) 14 connected to the OLT 10. The interface between the OpS 14 and the OLT 10 is defined by ITU-TQ.834.1. Here, the direction from the OLT to the ONUs is defined as downstream. The direction from the ONUs to the OLT is defined as upstream.
FIG. 4 shows a frame format in BPON. FIG. 4 shows, beginning at the top, a downstream frame format, an upstream frame format, and an enlarged portion of the upstream frame format. The OLT 10 sends 224 downstream ATM cells to the ONUs in one cycle, and sends a PLOAM (physical layer OAM) cell 241 to the ONUs 12 every 27 cells for the purpose of system control and the setting of upstream bandwidths. The ONUs 12 monitor connection identifiers VPI/VCI contained in cell headers of ATM cells 252 received from the OLT 10, get only cells directed to the pertinent ONUs 12, and send user data to physical lines 18 of the user side. The ONUs 12 transform user packets received from users 13 to ATM cells, append a header PON-OH (PON-OverHead) 251 to the leading portion of the ATM cells, and output the ATM cells to the OLT 10. The timing in which the ONUs 12 output the cells and the number of time slots are set in the system control cell PLOAM 241 sent from the OLT 10 and thereby specified to the ONUs by the OLT. Upstream cells sent to the OLT from the ONUs are multiplexed (e.g., time division multiplexing) on an identical optical fiber of the OLT side in the SC. To prevent cells sent from the ONUs from conflicting with each other in the SC, the OLT uses the PLOAM cell to make output timing adjustment called ranging and set timing in the ONUs. The OLT assigns time slots to the ONUs. In other words, time slot assignment in consideration of users under the ONUs is not performed.
A transfer method of a BPON system is defined by ITU-T G.983.1 and G.983.2. For EPON (Ethernet-PON) systems that perform a transfer between OLT and ONUs over Ethernet, IEEE 802.3ah is pushing ahead with standardization of transfer methods.
FIG. 6 shows a network configuration that allows users to connect to the Internet 30 in FTTH. This network comprises a broadband access server BAS 28 that performs aggregation of user accesses, user management, and service allocation; an OLT 10 that receives user data from the BAS and sends it to a PON system, and manages the PON system; SC 11 that branches a single optical fiber to plural optical fibers; and ONUs 12 that terminate user access and send user data to the PON system (OLT) according to the OLT. Users 13 are authorized in the BAS via the ONUs 12 and the OLT 10 before being connected to the Internet. In this configuration, the PON system is used as a communication path for connecting the ONUs installed within user premises and the BAS, wherein a bandwidth has been allocated to the communication path.
In BPON and EPON systems, a path between ONUs and OLT is used as a data path to which a bandwidth has been allocated, and bandwidth control between OLT and ONUs is performed in an ONU unit. As more and more users introduce FTTH, a PON system suffers from the problem of the number of branches of SC. Since one ONU is normally installed for each user, an optical fiber of one branch is required for one user. To accommodate more users in an identical PON system, the SC must be adapted to have more branches. The number of branches of the SC is limited by physical constraints attributed to laser output installed in both the OLT and ONU. Multiple branches are achieved only by use of very expensive and high-output lasers. Accordingly, one method for avoiding the limitation on the number of branches is to accommodate plural users in an ONU and share the ONU among the users. However, since the OLT and the BAS operate independently, in the prior art, the OLT has not performed bandwidth control for each of users under the ONUs.